In general, a variable-displacement vane oil pump comprises:                a main body presenting a cavity;        a mobile ring, accommodated in the cavity, within which there is, in turn, a hub adapted to turn about an axis; the hub is provided with a plurality of vanes, each of which is adapted to slide in a corresponding slot obtained in the hub itself.        
Normally, there is further provided a moving device of the mobile ring according to a driving pressure between a central position on the rotation axis of the hub towards a predetermined extreme position, eccentric with respect to the rotation axis of the hub itself.
Furthermore, in the specific variable-displacement vane pump described and claimed in international patent application WO 03/023228 (PIERBURG), the moving device comprises a driving chamber made on a wall of the cavity and an overhanging projection of the mobile ring. The projection is adapted to slide in the driving chamber due to a driving pressure present in the driving chamber itself. Furthermore, in the pump described in the aforesaid document, there is a guiding device for guiding the mobile ring in the cavity. Furthermore, the guiding device comprises a protrusion coupled to a guiding seat, and a spring arranged inside the guiding seat, which elastically acts on the mobile ring.
The text of the aforesaid patent application WO 03/023228 (PIERBURG) must be considered to all effects an integral part of the present description.
Currently, the contact between mobile ring and main body of the pump occurs through a prismatic contact (surface-on-surface). This means that there is no lubrication between the two surfaces because there is direct contact between them.
During the life of the pump and in highly loaded engine applications, a direct contact between surfaces may lead as an undesired collateral effect to an initial adhesive wear followed by a subsequent abrasive wear.
Although, actually, catastrophic events caused by the contact between surfaces in highly loaded engine applications of the variable eccentricity vane pump have not yet been reported, it appears appropriate to introduce the innovations object of the present description.
Incidentally, in this context, “highly loaded engine applications” mean the applications in which the pump is subjected to very fast rotation speeds and/or very high pressures (i.e. some sports car engines or some truck engines).